Fuel issues. To observe fuel flow, place a tube/hose on carb bowl nipple, place other end in a jar. Open bowl drain screw and observe fuel flow. Do this for both sides.
Methodical process of running down the problem. Remove petcock. Inspect, clean, reinstall. Remove and clean then replace the hidden filter. Find anything dirty/clogged yet? Yes? Then after cleaning it up try riding her again. No help? then on to dropping the carb bowls. Using a mirror inspect for debris clogging jets. No obvious material blocking jets? Drop jets. Blow out each jet. Do not use wire to clear out the jets.
How old is your sickle? Sometimes a very little bit of filth is all it takes to starve the jets. Given that she runs better at 2.5K rpms, it would appear that your pilot circuit is dirty. (usually throttle position is what counts not rpms. odd that one.) Pilot jets under the bowls and the air mixture screws outside the bowls. Do your research on this site for the components that comprise the air mixture assemblies. Place a towel on the top of your engine case to catch the tiny little o-ring , washer and spring that are part of that air mixture screw assembles.
If this fails, you may have to blow out the internal low speed circuits of each carb. Also you may want to check the float height. That same tube/hose you placed on the carb bowl nipple , if clear, can be used to do that.
